1. Field of the Invention
The present invention relates to the field of data communications. More specifically, the present invention relates to synchronization in time-division multiple access systems.
2. Description of Related Art
In systems that employ Time Division Multiple Access (TDMA) schemes for sharing a radio spectrum between several simultaneous communications, such as Global System for Mobile Communications (GSM), synchronization of clocking mechanisms is vital to their reliable operation. In TDMA as utilized in GSM, a given frequency bandwidth carries information in a number of separate "slots," with each slot carrying a single "burst" which is the minimum transmission unit. A series of slots composes a frame, and it is with the unit of the frame that synchronization must be achieved.
In a GSM, a base station sub-system (BSS) contacts a multitude of mobile stations using a radio interface, and thereby maintains a communication pathway to the mobile stations. Typical examples of mobile stations (MS) include hand-held cellular telephones and mobile cellular telephones (car phones). Mobile stations must synchronize with their base stations after the mobile stations are powered up such that the mobile station and base station will be referring to the same frame number when communicating. However, due to the propagation delay inherent in communicating the frame between the MS and the BSS, a frame number assigned at the BSS does not correspond to the frame number generated by timing circuits at the MS. Further, when a frame number is decoded at the MS for use by a controller, which is a part of the mobile station, there may be an additional time lag before the controller can recognize the frame number.
One solution to synchronize and account for propagation delay is to set timers in real-time fashion on Digital Signal Processors within the MS. This solution fails if a controller, which must separately communicate with the DSP to obtain the frame number, is to be synchronized with the BSS. The extra time lag from the DSP to the controller must be taken into account if the controller is to handle functions related to message frames traditionally handled instead by the DSP. The controller must be synchronized with the base station if the controller carries out the timing (i.e. frame number generation) rather than the DSP.
Thus, a solution is needed that can perform synchronization in a non-real-time fashion on the controller rather than the digital signal processor.